Masthead Sailing sells an "outhaul kit" - includes 2 clam cleats and the 2 lines required. $32 is cheaper than just buying 2 of the aluminum cleats from one of the Sunfish dealers.
(this previous post has some additional info)
until you find the leaks, draining it after each use is your best bet. heck, draining it after each use is just good practice anyway.
the heavy waterlogged hulls are I think normally due to neglect and long exposure to water (either through sailing, or exposure to the elements).
L&VW mentioned it and others on the Sunfish Forum have used a piece cut from one of those hard plastic cutting boards.
I personally think if you can get a fender washer on 2 of the 3 screws, you'd probably be fine enough, especially if the 3rd straddled the deck-to-cockpit wall joint, which...
I've never tried the rubber washers, but it might not hurt to add them.
I just go to a local hardware store and pick up a couple stainless steel machine bolts to match the eyestrap holes. Under the deck, I use the bigger washers (might be called fender washers?) and a nylon locking nut.
Sunfish guy here, but I've used two different mount setups on my Sunfish.
home-made mount on the tiller:
with example screenshot:
and also mounted w/ a standard GoPro mount out on the back end of the boom:
I pretty much exclusively mount it on the boom now - gets pretty decent videos...
if you don't trust the double ziploc bag method, there are many options for small dry-bags for your phone.
they are waterproof, and you can often usually still operate the phone through the plastic sides of the case.
I've had good luck with ones like this from Aquapac.
waterproof phone case...
only reason I said to upgrade the mast cap is that you had said "Also will need to change the plastic piece on top of the mast that the halyard goes into".
the mainsheat cam cleat you linked to is often used on Sunfish. I personally prefer the ratchet block setup - and if you block with the...
I'd highly recommend upgrading from the hook. Get a ratchet block and a cleat if you want that option (the cam cleat is not necessary, but can help it lighter winds to hold the mainsheet).
this is my preferred setup (shown on a Minifish...)
on a Sunfish, I mount the Harken 150 cleat right...
I believe that the holding ratio (10:1, 15:1, 20:1, whatever it may be...) is the maximum based on the most contact of the sheet with the sheave (180-degrees per the image below), so in most practical cases on a Sunfish, you'd be hiking out with the sail close hauled or on some reach, and that...